Unscrew the yoke screw fully and remove the dust cap
Using a 3/4 inch box wrench, unscrew the nut (yoke connector) that holds the yoke into the reg body
Keep the conical filter that is inside the yoke connector. You'll need to put it in the DIN connector.
Install a new o-ring into the face of the DIN connector (the face end has a hex shaped hole in it and the o-ring may already be new)
Insert the conical end of the filter into the threaded end of the DIN connector
Install a new lightly lubricated o-ring into the end (threaded end) of the DIN connector around the filter
Insert the DIN connector through the hand wheel so that the face (end with the hex hole) is on the threaded side of the handwheel.
Holding the reg body with the inlet hold down, insert the threaded part of the DIN connector up into the reg body (so that the filter doesn't fall out) and tighten finger tight
(At this point, the pros mount the reg body to (not in) a vice using a reg servicing tool. I wouldn't recommend clamping the reg body itself into a vice because it's made of brass and will deform easily. You can pretty much do the same by holding it firmly in your hand)
Use a 6mm hex key to tighten the DIN connector. It's supposed to be tightened to 12 foot pounds, with a torque wrench. That is NOT very tight. Be cautious as the body of the reg is made of brass and the threads will strip easily.
If ANY of this sounds a little beyond you, then take your reg to a shop and have them do it. I can't see them charging you more than a few bucks, if at all.
If you do a little web searching, you can find pdf copies of the ATX 200 first stage maintenance manual. It's pretty typical of most of the Apeks first stages.
BTW, I'm not a reg tech. Someone helped me to do this a few years ago and I'm just returning the favor.